<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			body {
				display: flex;
				justify-content: center;
				flex-direction: column;
				align-items: center;
			}

			.title {
				color: greenyellow;
			}

			.main {
				position: relative;
				width: 400px;
				height: 200px;
				border: 1px solid red;
			}

			.main span:nth-child(1) {
				position: absolute;
				left: -5px;
				top: -5px;
				padding: 15px;
				border-style: solid;
				border-color: rebeccapurple;
				border-width: 5px 0 0 5px;
			}

			.main span:nth-child(2) {
				position: absolute;
				right: -5px;
				top: -5px;
				padding: 15px;
				border-style: solid;
				border-color: rebeccapurple;
				border-width: 5px 5px 0 0;
			}

			.main span:nth-child(3) {
				position: absolute;
				right: -5px;
				bottom: -5px;
				padding: 15px;
				border-style: solid;
				border-color: rebeccapurple;
				border-width: 0 5px 5px 0;
			}

			.main span:nth-child(4) {
				position: absolute;
				left: -5px;
				bottom: -5px;
				padding: 15px;
				border-style: solid;
				border-color: rebeccapurple;
				border-width: 0 0 5px 5px;
			}
		</style>
	</head>
	<body>
		<h1 class="title">css实现矩形边角加粗</h1>
		<div class="main">
			<span></span>
			<span></span>
			<span></span>
			<span></span>
		</div>
	</body>
</html>
